Projection objective of a microlithographic projection exposure apparatus
View Patent ↗A projection objective of a microlithographic projection exposure apparatus comprises a manipulator for reducing rotationally asymmetric image errors. The manipulator in turn contains a lens, an optical element and an interspace formed between the lens and the optical element, which can be filled with a liquid. At least one actuator acting exclusively on the lens is furthermore provided, which can generate a rotationally asymmetric deformation of the lens.
1. An apparatus, comprising:
a first optical element that is refractive;
a second element;
a liquid between the first optical element and the second element; and
an actuator coupled to the first optical element such that operation of the actuator causes a rotationally asymmetric deformation of the first optical element substantially without causing a deformation of the second element,
wherein the apparatus is a microlithographic projection exposure apparatus.
2. An apparatus according to claim 1 , wherein the first optical element is a plane-parallel plate.
3. An apparatus according to claim 1 , wherein the first optical element is a meniscus lens.
4. An apparatus according to claim 1 , wherein the first optical element is a membrane.
5. An apparatus according to claim 1 , wherein:
there is an interspace between the first optical element and the second element and, the interspace has, in a direction along an optical axis of a projection objective of the apparatus, a maximum thickness of less than 1 mm.
6. An apparatus according to claim 5 , wherein the maximum thickness is less than 2 μm.
7. An apparatus according to claim 1 , wherein the deformation of the first optical element has an m-fold symmetry or a superposition of a plurality of m-fold symmetries with m being an even, positive integer.
8. An apparatus according to claim 1 , comprising at least two actuators that are configured to exert bending moments on the first optical element.
9. An apparatus according to claim 8 , wherein the at least two actuators are arranged along a circumference of the first optical element.
10. An apparatus according to claim 8 , wherein the actuators are configured such that at least two deformations of the first optical element can be generated having a different m-fold symmetry.
11. An apparatus according to claim 1 , wherein the second element is reflective.
12. An apparatus according to claim 1 , wherein the second element is refractive.
13. An apparatus according to claim 12 , wherein the second element is a meniscus lens or a plane-parallel plate.
14. An apparatus according to claim 1 , comprising a second actuator configured to act exclusively on the second element to generate a deformation of the second element.
15. An apparatus according to claim 14 , wherein the deformation of the second element is rotationally asymmetric.
16. An apparatus according to claim 14 , wherein the deformation of the second element is rotationally symmetric.
17. An apparatus according to claim 1 , comprising a pressure equalizer configured to maintain a constant pressure of the liquid in an immersion space.
18. An apparatus according to claim 1 , wherein the first optical element is arranged in or in close proximity of a pupil plane.
19. An apparatus according to claim 1 , wherein the actuator is fastened to a front or a rear surface of the first optical element, and wherein the actuator is configured to produce on the first optical element compressive and/or tensile forces along directions that are at least substantially tangential to the surface.
20. An apparatus according to claim 19 , wherein the actuator is formed by a layer which is applied to the surface, the layer having a variable dimension along at least one direction.
21. An apparatus according to claim 20 , wherein the layer comprises a piezo electric material.
22. An apparatus according to claim 20 , wherein the layer comprises a material having a coefficient of thermal expansion that is different from the coefficient of thermal expansion of the first optical element.
23. An apparatus according to claim 22 , comprising a device configured to controllably change the temperature of the layer.
24. An apparatus according of claim 23 , wherein the device is configured to direct radiation onto the layer.
25. An apparatus according of claim 23 , wherein the device is configured to apply a voltage to the layer.
26. An apparatus according to claim 20 , wherein a plurality of layers have the shape of ring segments, the ring segments forming a circular ring which is centered with respect to an optical axis of a projection objective of the apparatus.
27. An apparatus according to claim 26 , wherein the ring segments are spaced apart by gaps.
28. An apparatus according claim 19 , wherein a plurality of actuators are distributed over the surface within an area through which projection light propagates.
29. An apparatus according to claim 28 , wherein the plurality of actuators are at least substantially transparent at the wavelengths of the projection light.
30. An apparatus according to claim 28 , wherein the plurality of actuators are opaque at the wavelengths of the projection light, and wherein the manipulator is arranged in or in close proximity to a pupil plane of a projection objective of the apparatus.
31. An apparatus according to claim 1 , comprising a sensor that is fastened to a front or rear surface of the first optical element, wherein the sensor is configured to measure a deformation of the first optical element produced by the actuator.
32. An apparatus according to claim 31 , wherein the sensor is configured to produce an output signal which depends on the amount of compressive and/or tensile forces produced in the sensor by a deformation of the first optical element.
33. An apparatus according to claim 1 , wherein a plurality of actuators are distributed over an area, through which projection light is allowed to propagate, of a surface of the first optical element which is opposite to a surface which is in contact with the liquid.
34. An apparatus according to claim 33 , comprising a further optical element that is refractive on which the plurality of actuators rest.
35. An apparatus according to claim 1 , further comprising:
a third optical element; and
means for deforming the third optical element,
wherein there is an interspace between the second element and a third optical element, both the second element and the third optical element are refractive.
36. An apparatus according to claim 1 , comprising a device configured to change the pressure of the liquid.
37. An apparatus according to claim 1 , wherein the first optical element has a shape in its undeformed state which is rotationally asymmetric.
38. An apparatus according to claim 37 , wherein the first optical element has a thickness distribution in its undeformed state which is rotationally asymmetric.
39. An apparatus according to claim 38 , wherein the first optical element has a first surface, which is in contact to the liquid and has a rotationally asymmetric shape, and a second surface opposite the first surface, wherein the second surface has a rotationally symmetric shape.
40. An apparatus according to claim 1 , further comprising:
a controller configured to drive the actuator; and
a sensor arrangement connected to the controller, the sensor arrangement being configured to determine the image errors.
41. The apparatus of claim 40 , wherein the sensor arrangement comprises a CCD sensor, which is positionable in an image plane of a projection objective of the apparatus or in a field plane conjugate therewith.
42. The apparatus of claim 41 , wherein the actuator, the controller and the sensor arrangement form a closed feedback loop.
43. A method, comprising:
providing an apparatus according to claim 1 , the apparatus comprising an illumination system and a projection objective;
arranging a mask in an object plane of the projection objective of the apparatus;
using the illumination system to illuminate the mask with light; and
after illuminating the mask with light, using the projection objective to project the mask onto a photosensitive layer which is arranged in an image plane of the projection objective.
44. An apparatus, comprising:
a first optical element that is refractive, the first optical element having a thickness distribution in its undeformed state which is rotationally asymmetric,
a second element,
a fluid between the first optical element and the second element; and
means for deforming the first optical element,
wherein the apparatus is a microlithographic projection exposure apparatus.
45. A method, comprising:
providing an apparatus according to claim 44 , the apparatus comprising an illumination system and a projection objective;
arranging a mask in an object plane of the projection objective of the apparatus;
using the illumination system to illuminate the mask with light; and
after illuminating the mask with light, using the projection objective to project the mask onto a photosensitive layer which is arranged in an image plane of the projection objective.
46. An apparatus, comprising:
a first optical element that is refractive;
a second element;
a liquid between the first optical element and the second element; and
an actuator coupled to the first optical element such that operation of the actuator causes a rotationally asymmetric deformation of the first optical element substantially without causing a deformation of the second element,
wherein a ratio between the refractive index of the first optical element and the refractive index of the liquid is between 0.9 and 1.1, and the apparatus is a microlithographic projection exposure apparatus.
47. An apparatus according to claim 46 , wherein the ratio between the refractive index of the first optical element and the refractive index of the liquid is between 0.99 and 1.01.